Question

I have a generator function getElements in a class Reader() that yields all the elements out of an xml file. I also want to have a function getFeatures that only yields the elements with a feature tag.

How I tried it is to have a flag featuresOnly that is set to True when getFeatures is called, and in getFeatures call self.getElements, like this:

def getFeatures(self):
    self.getFeaturesOnly = True
    self.getElements()

This way in getElements() I only have to do

def getElements(self):
    inFile = open(self.path)
    for element in cElementTree.iterparse(inFile):
        if self.getFeaturesOnly == True:
            if element.tag == 'feature':
                yield element
        else:
            yield element
     inFile.close()

However, when I do this and run it

 features = parseFeatureXML.Reader(filePath)
 for element in features.getFeatures():#
       print element

I get: TypeError: ‘NoneType’ object is not iterable
This is because getFeatures doesn’t contain a yield. Now, the way that I know how to solve this is to copy the code of getElements into getFeatures and only use the

if elementFunctions.getElmentTag(element) == 'feature':

in the getFeatures() function, but I rather not duplicate any code. So how would I be able to keep on generator function, and have a different function where I only specefy which tag I would like to get?

Answer 1

First things first: You have that error because you don’t return the generator

Meaning that you have to change:

def getFeatures(self):
    self.getFeaturesOnly = True
    self.getElements()

with:

def getFeatures(self):
    self.getFeaturesOnly = True
    return self.getElements()    # returning the generator

Cleared this, TBH I wouldn’t design my Reader() class like this.

I’d let the getElement yield all the elements:

def getElements(self):
    inFile = open(self.path)
    for element in cElementTree.iterparse(inFile):
        yield element
    inFile.close()

And then getFeatures() do the filtering:

def getFeatures(self):
    for element in self.getElements():
        if element.tag == 'feature':
            yield element